Bluetooth: ATT: Fix using nano_delayed_work API

This fixes warnings related to the use of nano_work and
nano_delayed_work in ATT layer.

Note that k_delayed_work takes a timeout in miliseconds rather than in
ticks thus the timeouts values changes.

Change-Id: I14d8438c1537febcb7768ef2934042ce38682739
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>